The Venti, also called anemoi thuellai in Greek, are known as storm spirits. Aeolus locked them away on his island because they are creatures of chaos. How they were created was when Zeus defeated Typhon. In Percy Jackson's THe Heroes of Olympus, they are known to serve Gaea.
Storm spirits are known to have two forms; depending on how chaotic they are, their forms change. They can appear as horses or humans. For when they're a horse, their appearance is of electric manes and storm-cloud bodies. When they're human, or human-like, their appearance is of a boy with electric sparkling eyes and smoky wings.